1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that forms an image on a recording material, for example, a copying machine, a printer, or a facsimile machine.
2. Description of the Related Art
In electrophotographic image forming apparatuses, a toner formed on a recording material is fixed as a permanent image by a fixing device.
Among various types of fixing devices, a belt fixing device using a belt has attracted attention as a device that meets a recent demand to increase the image forming speed. This type of fixing device is disclosed in Japanese Patent Laid-Open No. 61-132972.
In this belt fixing device, the length of a fixing nip in the traveling direction of a recording material can be increased by the use of the belt, and more heat can be applied to a recording material on which an unfixed toner image is formed than before. That is, the belt fixing device has a great advantage in its capability of applying a sufficient amount of heat to the recording material even when the image forming speed, in other words, the conveying speed of the recording material increases.
When a highly glossy resin coated paper in which a paper base material is coated with acrylic resin or polyolefin resin (hereinafter referred to as coated paper) is used as a recording material, an image defect that partially raises the coated paper (hereinafter referred to as a blister) sometimes occurs.
A blister seems to occur for the following reason. That is, when excessive heat is applied from the belt to the back side of coated paper, moisture in the base material evaporates and the volume of the coated paper increases. The vapor is intensively radiated from a thin portion or a vacancy of the coating layer. When this phenomenon is marked, a part of the coating layer is sometimes torn.
Accordingly, in a fixing device disclosed in Japanese Patent Laid-Open No. 11-194647, a belt is separated from a fixing roller during standby so that the increase in temperature of the belt is prevented and excessive heat is not applied from the belt to the back side of coated paper.
However, it is difficult to separate the belt from the fixing roller during a continuous copying operation, and in addition, the length of the fixing nip is increased. Therefore, the temperature of the belt increases with the progress of the continuous copying operation.
In the above-described known belt fixing device, in order to increase image productivity, an image forming job is performed immediately after the completion of the previous image forming job, although the temperature of the belt has increased during the previous image forming job.
That is, even when coated paper is used in the image forming job subsequent to the previous image forming job, priority is given to image productivity. Therefore, there is room for improvement in the image quality.
In the market for electrophotographic apparatuses, operators that give priority to image productivity and operators that give priority to image quality are mixed.
However, since known belt fixing devices are intended to give priority to image productivity, they can satisfy the operators that give priority to image productivity, but cannot satisfy the operators that give priority to image quality.
According to the image forming circumstances, an operator sometimes gives priority to image quality, and sometimes gives priority to image productivity. The known belt fixing devices do not meet these various demands of the operators.